Food Selection by Silkworm Larva

Silkworm (bombyx mori) is a phytophagous insect. Generally, phytophagous insects have a certain degree of selectivity for the plants they eat. In addition to eating mulberry leaves, Bombyx mori can still feed on moraceae, dandelions and lettuce in the Asteraceae family, and wild elm in the Ulmus family. However, the leaves of plants other than mulberry can hardly make the silkworm grow and reproduce healthily. It can be seen that the silkworm chooses mulberry leaves as food more strictly. It not only selects edible plants, but also means that after eating the plants, it can meet the nutrients needed for its growth, and the individual can fully develop and reproduce. Offspring.

The choice of food for animals is generally achieved through the visual organs, but the monocular structure of the silkworm larvae cannot image objects on the omentum cells, so they cannot identify objects, only the strength and wavelength of light. Therefore, the food selection of silkworms mainly depends on the organs of smell and taste. Herbivorous insects select host plants by identifying secondary metabolites unique to plants. The chemical substances identified by insects include glycosides, tannins, alkaloids, volatile aromatic oils, and organic acids. Some of them act as feeding-promoting substances to make insects eat, and some act as feeding-inhibiting substances to make insects anti-feeding.

If the 5th instar normal silkworms and the silkworms with their antennae removed are placed 10cm away from the mulberry leaves, after a few minutes, almost all normal silkworms can be seen crawling on the mulberry leaves, while a few of the silkworms with their antennae removed accidentally reach the mulberry while crawling. Outside of the leaves, almost none of them can reach the mulberry leaves. It can be seen that the odor of the leaves is an important factor for silkworms to reach the leaves of food, and it is the olfactory organ-the antennae that senses the odor. Mulberry mulberry leaves odor is formed in a class of volatile substances, such as beta], gamma] – hexyl alcohol, [alpha], [beta] hexenal, citral, linalool, linalyl acetate and the like. This kind of substance can induce appetite and make silkworms have eating action, which is called attractant factor. The attractant factors not only exist in mulberry leaves, but also widely exist in other plants. For example, silkworms also have a strong tendency towards peach leaves.

When silkworms are attracted to the leaves by stimulating their appetites, whether they bite or not depends on the presence or absence of the biting factors. The chemical substances in mulberry leaves that cause silkworm bites are

β- sitosterol, morin, quercetin isobutyl phlorizin like. Phellin and isoquercitrin are unique substances of mulberry. β-sitosterol is contained in the waxy layer of the epidermis of mulberry leaves. It is easy to wipe it out with absorbent cotton soaked with ether. With filter paper dipped in β-sitosterol, silkworms flock and bite. The biting factor is sensed by the lower jaw. After the lower jaw is removed, the silkworm loses the ability to discriminate and also bites on plant leaves that are not fed under normal conditions.

In addition to the two factors mentioned above, the swallowing factor is needed to cope with the continuous feeding action of the silkworm. The main substance that promotes swallowing is cellulose. The main function of cellulose is to stimulate the peristalsis of the digestive tract, so that the food tablets are continuously transported to the back of the digestive tract. In addition, sucrose, inositol, phosphate, silicate, vitamin C, chlorogenic acid, etc. also promote the swallowing of mulberry leaves.

After adding citral, β-sitosterol, molin, sugar, cellulose and phosphate to artificial feed , silkworms are first attracted by the citral in the feed and bite due to the presence of β-sitosterol. The presence of Phellinin makes the bite more vigorous, and sugar, cellulose and phosphate promote its continuous swallowing. From the perspective of food intake, it is almost as greedy as eating mulberry leaves.

In fact, the three factors that cause silkworms to eat are not unique to mulberry leaves and a few plants. They are also present in many other plants, but silkworms do not feed on these plants. Experiments have shown that the food selectivity of silkworms depends not only on the presence or absence of three feeding factors, but also on whether they contain appetite-suppressing repellent factors. The so-called repellent factors are mainly alkaloids with a bitter taste. There are sensory cells that sense alkaloids on the mandibular tumor of silkworms.

There are two nodular sensory protrusions Ss-I and Ss-II on the mandibular body of silkworm. Ss-I is distributed with sugar-sensing cells that can sense carbohydrates, especially sucrose, and inositol-sensing cells that sense inositol, as well as salt-sensing cells and acid-sensing cells that can sense salt and acid. Ss-II is distributed with bitter substance sensing cells that sense bitter substances such as alkaloids, water sensing cells that sense dehydration, and other sensing cells. The silkworm perceives the concentration of various foods that promote food intake and inhibits food intake through these sensory cells, and transmits this information to the central nervous system, and then determines whether to eat after analysis and synthesis. Mulberry leaves contain a large amount of sucrose and inositol that maximize the activation of sugar and inositol-sensing cells, and do not contain substances that activate bitter taste-sensing cells, so silkworms have a special preference for mulberry leaves. To most other plant leaves, the sugar and inositol sensing cells react weakly, even one or both of them do not respond, and the bitter substance sensing cells are activated, so most plants cannot cause silkworm ingestion.